Transport Processes in Wavy Walled Channels

Abstract

Due to their complex shape, wavy walled geometries are capable of inducing unsteady and even chaotic flows at low Reynolds numbers. The convective effects of the unsteady motion significantly enhances heat and mass transport in the fluid. Because of this, wavy walled channels are commonly used in applications such as heat exchangers. Despite their common use however, a systematic investigation of the dependence of the fluid flow and heat transfer on the geometric parameters of the channel does not exist. In many heat exchanger applications, the working fluid contains suspended particulates. When cooling these particle laden flows, thermophoretic forces induced on the particles by thermal gradients in the fluid result in their deposition along the cooler walls. This process, known as fouling, leads to the formation of a porous layer, which reduces the effectiveness of the cooler. One application in which fouling is a significant issue is exhaust gas recirculation (EGR) used in diesel and gasoline engines to reduce nitrogen oxides (NOx) emissions. The heat exchanger used in this process experiences rapid degradation in performance from fouling caused by the high concentration of soot particles entrained in the exhaust gas. Recently, engine manufacturers have begun using wavy walled heat exchangers as empirical evidence suggests that this geometry is less prone to fouling. However, a limited amount of research has been performed to understand how this geometry reduces fouling and the dependence of this reduction on the geometric parameters of the channel. In this work we use computational modeling to investigate the effect of asymmetric wavy walled channel geometries on laminar fluid flow and heat/mass transfer. To this end, we develop a computational model based on the lattice Boltzmann method, explicit finite differences and Brownian dynamics to simulate unsteady viscous flow and heat/mass transport in wavy walled channel geometries and use this model to systematically examine these processes. Furthermore, we investigate the formation of deposit layers resulting from thermophoretic deposition of particulates transported by the flow onto the channel walls and probe how this process can be mitigated using a wavy wall geometry. The results from our studies are important for designing laminar heat/mass exchangers utilizing unsteady flows for enhancing transport processes. Additionally these results provide valuable information necessary to develop heat exchangers which are less prone to fouling.